1. Field of the Invention
The present invention relates to an optical scanning device, an optical writing device, and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus such as a digital copier and a laser printer forms a latent image by radiating a light beam to a photosensitive element, which is an image carrier, using an optical scanning device. Recently, high-density image formation through optical scanning is developing, and a smaller beam-spot diameter on the photosensitive element is demanded of the image forming apparatus.
In addition to the high density, high-speed image formation is progressing, and an optical scanning device meeting these requirements is demanded. As a means of increasing the speed and the density, there is a means of causing a polygon mirror as a deflecting unit to rotate at high speed. The means, however, has some problems such as increase in power consumption and noise, heat generation, and degradation of its durability.
As for the optical scanning device, it is desirable to promote resin lenses to target cost reduction.
The resin lenses, however, have large fluctuations of a surface curvature, a thickness, and a refractive index of lenses due to change in environmental temperature, and also have large changes in the refractive index due to change in wavelength of a semiconductor laser being a light source, as compared with those of glass lenses. The resin lenses are therefore largely affected by these changes.
The glass lenses also have fluctuations of a surface curvature, a thickness, and a refractive index of lenses due to change in environmental temperature, and also have change in a focus position caused by changes in the refractive index due to the change in wavelength of a semiconductor laser which is a light source. A spot diameter thereby increases, which causes image degradation.
There are various kinds of proposals to solve these problems.
For example, Japanese Patent Application Laid-Open No. 2004-126192 proposes a technology for reducing the change in a focus position due to temperature variation by combining a diffractive surface with a refractive surface of an optical system located upstream of a deflector. In the conventional technology, however, necessary consideration is not given to the change in the focus position due to temperature of the optical system (scanning optical system) located downstream of the deflector. Moreover, there is no reference to multibeam that is a plurality of beams for scanning one photosensitive element.
Japanese Patent Application Laid-Open No. 2003-337295 and Japanese Patent Application Laid-Open No. H11-223783 propose a technology for correcting the change in the focus position caused by a scanning optical system due to temperature variation, by the change in power of a diffractive portion. However, consideration is not given to the change in arrangement of optical elements in a first optical system. Moreover, there is no reference to multibeam that is a plurality of beams for scanning a same photosensitive element.
Japanese Patent Application Laid-Open No. 2002-214556 proposes a technology for a scanning/imaging optical system, an optical scanning device, and an image forming apparatus, and a method of correcting the change in a focus position due to temperature by combining at least three lenses in an optical system located upstream of a deflector without using a diffractive surface.
Japanese Patent Application Laid-Open No. 2005-258392 proposes a technology for reducing the change in a focus position due to temperature variation by combining a diffractive surface and a refractive surface of an optical system located upstream of a deflector. Although multibeam is mentioned in this conventional technology, two diffractive surfaces are provided on the upstream of the deflector.
As a means of solving the problem, as the conventional technology described in Japanese Patent Application Laid-Open No. 2002-214556, there is the method of correcting the change in a focus position by combining at least three lenses in the optical system located upstream of the deflector. In this method, however, an increase in the number of lenses results in an increase in cost. Even in this case, a piece of glass lens is required, which also causes the increase in cost.
Japanese Patent Application Laid-Open No. H10-333070 discloses a method of correcting the change by providing a diffractive optical surface on a scanning lens. However, the scanning lens has a wide range through which a light flux passes, and requires time for machining the diffractive optical surface, which causes an increase in cost.
To solve the problems, the conventional technologies such as Japanese Patent Application Laid-Open No. 2004-126192, Japanese Patent Application Laid-Open No. 2003-337295, and Japanese Patent Application Laid-Open No. H11-223783 propose the methods of reducing the change in beam spots due to temperature variation by using the resin lens with the diffractive surface located upstream of the deflector.
The conventional technologies, however, have a problem specific to the multibeam. The problem is such that a wavelength difference between beams is not mentioned, and that even if performance of one beam can be acquired, performance of another beam cannot be acquired. For high image quality, there remains a problem of a difference in the amount of light between multibeams.
There is known an optical scanning device (laser scanning device) described in Japanese Patent Application Laid-Open No. 2002-287062. The optical scanning device employs a diffractive surface to make its optical property stable, allowing for the change in the optical property due to the temperature variation and for the wavelength change in a light source.
The optical scanning device disclosed in Japanese Patent Application Laid-Open No. 2002-287062 is such that a light-source optical system is configured with one resin-made optical element. More specifically, the light-source optical system causes a laser light emitted from a laser light source to be a parallel light in a main scanning direction and causes the laser light to be collected near a deflection/reflection facet of an optical deflector in a sub-scanning direction. The optical element has one or more reflection surfaces having no rotationally symmetric axis and also has two transmission surfaces, and provides a diffractive surface in each transmission surface. This patent application also discloses, as a comparative example, an optical scanning device in which a diffractive surface is provided in each of a resin-made collimator lens for collimating a light beam emitted from a semiconductor laser and of a resin-made cylinder lens for focusing the collimated light beam in the sub-scanning direction. The “diffractive surface” has a lens power due to diffraction.
The light-source optical system disclosed in Japanese Patent Application Laid-Open No. 2002-287062 has to form the transmission surface and the reflection surface in one optical element, and includes a curved reflection surface. Therefore, the light-source optical system is not always easy to be manufactured, and thus, there is room for improvement in terms of cost reduction of the optical scanning device.
Furthermore, the diffractive surface generally requires micromachining technology, and also requires extremely high precision. For example, the diffractive surface having power and a focal length f equivalent to those of a spherical lens as shown in FIG. 23A is in a shape as shown in FIG. 23B. This is a shape formed so that the height of the spherical surface is made even with respect to a substrate. As is clear from FIG. 23B, the diffractive surface has grooves whose pitches are becoming narrower with distance from the optical axis, which causes machining to become exponentially difficult. Furthermore, any diffractive surface sandwiched by back cuts needs to be part of the spherical surface. Although the part of the spherical surface can be approximated as a straight line, diffraction efficiency inevitably decreases in this case. However, if the diffractive surface is formed so as to be part of the spherical surface, roughness of its surface shape becomes noticeable, which causes degradation of wavefront aberration, and the beam-spot diameter thereby increases. And, occurrence of scattered light causes ghost to occur and transmission efficiency of light to decrease.